1. Field of the Invention
This invention relates to electrochemical processes and, in particular, to electrolysis.
2. Art Background
Presently, electrical generation in a primary power facility must be varied in accordance with the power demands at a particular time of day. This variation is obviously inefficient and causes additional wear on equipment such as generators. Various forms of energy storage have been suggested to reduce this inefficiency and equipment wear. For example, it has been proposed that the energy generation be maintained at a constant level throughout the day and that excess energy produced at any particular time be stored.
One proposed method employs this excess energy to produce hydrogen through electrolysis. The hydrogen gas thus produced is stored and combusted when desired to regain energy. Since hydrogen gas is easily stored without loss of potential energy, this method mitigates some of the problems associated with other methods of power storage, e.g., use of batteries.
Because of the potential advantages of hydrogen energy storage, and because of the interest in water electrolysis as it relates to fuel cells, the electrolysis of water has been an extremely well studied electrochemical reaction. Generally, the efficiency of the anode reaction, i.e., 4OH.sup.- .fwdarw.2H.sub.2 O+O.sub.2 +4e.sup.- rather than the hydrogen producing cathode reaction has limited the efficiency of the cell and thus the efficiency of H.sub.2 production. The efficiency for such a half cell reaction is generally reported by quoting an overvoltage required to produce a given current density, typically 100-1000 mA/cm.sup.2. Overvoltage is the potential above the thermodynamical value required to produce electrolysis at a given rate for a given temperature, pressure, and pH of the electrolyte. (See Bockris and Reddy, Modern Electrochemistry, page 883, (Plenum) New York, 1970.) Typically, overvoltages of approximately 1 volt are necessary to produce a current density of 100 mA/cm.sup.2.
Despite the efforts to develop a more efficient anode, the efficiency of electrolysis obtainable with a particular anode material is extremely unpredictable. Indeed, efficiencies vary appreciably with the electrode used. For example, when an elemental iridium anode is utilized, a voltage of about 2.1 volts versus a reversible hydrogen electrode (RHE), i.e., a 0.87 volt overpotential, at room temperature in 0.5 M H.sub.2 SO.sub.4 is necessary to produce a 100 mA/cm.sup.2 current density (and thus a corresponding rate of H.sub.2 production). In contrast, when an elemental Pt anode is utilized under the same conditions, 3.0 volts versus RHE, i.e., 1.77 overvoltage, are necessary to produce the same current density.
Other difficulties also arise in attempting to design a cell for electrolysis of water. Generally electrolysis will not occur in a non-conductive medium. The requisite level of conductivity in water is usually achieved by adding ionic compounds that produce either acidic or basic solutions. However, efficiency varies with the acidity or basicity of the water, i.e., the pH of the water. For example, an elemental iridium anode is most efficient in acid solution, e.g., in the pH range 0 to 3. In this range it requires approximately 2.1 volts versus RHE, i.e., 0.87 volts overvoltage, to produce a current density of 100 mA/cm.sup.2 at a pH of 0. In contrast, nickel is more efficient in basic solutions--1.5 volts versus RHE, i.e., 1.1 volts overvoltage, yields 100 mA/cm.sup.2 at a pH of 14. Similarly, NiCo.sub.2 O.sub.4 is even more efficient in basic solutions. With this anode 1.69 volts versus RHE (overvoltage of 1.29 volts) applied yields 250 mA/cm.sup.2 at a pH of 14.7.
To add to the complications and unpredictability introduced by electrode composition effects and by pH, other difficulties arise. For example, basic or acidic solution can adversely affect electrode stability. In fact, anodic iridium oxide anodes, although initially yielding good efficiency, after approximately 20 minutes for typical conditions rapidly decay due to dissolution of the iridium oxide. (See Gottesfeld and Srinivasan, Journal Electroanalytical Chemistry, 84, 117 (1977).)